Intermittent test failure: `TestNotifier/MultipleSubscribersStress`

[brandur]

Not super common, but I've seen it a couple times so far.

--- FAIL: TestNotifier (0.00s)
    --- FAIL: TestNotifier/MultipleSubscribersStress (0.25s)
        logger.go:[22](https://github.com/riverqueue/river/actions/runs/8257741142/job/22588819288?pr=258#step:8:23)5: time=20[24](https://github.com/riverqueue/river/actions/runs/8257741142/job/22588819288?pr=258#step:8:25)-03-13T00:59:07.700Z level=INFO msg="Notifier: Listener connecting"
        logger.go:225: time=2024-03-13T00:59:07.700Z level=INFO msg="Notifier: Run loop started"
        logger.go:2[25](https://github.com/riverqueue/river/actions/runs/8257741142/job/22588819288?pr=258#step:8:26): time=2024-03-13T00:59:07.700Z level=INFO msg="Notifier: Notifier healthy"
        notifier_test.go:425: Sending notification on "test_topic1": msg0
        logger.go:225: time=2024-03-13T00:59:07.701Z level=INFO msg="Notifier: Listening on topic" topic=test_topic1
        notifier_test.go:425: Sending notification on "test_topic1": msg1
        logger.go:225: time=2024-03-13T00:59:07.892Z level=INFO msg="Notifier: Unlistening on topic" topic=test_topic1
        logger.go:225: time=2024-03-13T00:59:07.892Z level=INFO msg="Notifier: Listener closing"
        logger.go:225: time=2024-03-13T00:59:07.893Z level=INFO msg="Notifier: Run loop stopped"
        notifier_test.go:462: Channel  0 contains   2 message(s)
        notifier_test.go:462: Channel  1 contains   2 message(s)
        notifier_test.go:462: Channel  2 contains   2 message(s)
        notifier_test.go:462: Channel  3 contains   2 message(s)
        notifier_test.go:462: Channel  4 contains   2 message(s)
        notifier_test.go:462: Channel  5 contains   2 message(s)
        notifier_test.go:462: Channel  6 contains   2 message(s)
        notifier_test.go:462: Channel  7 contains   0 message(s)
        notifier_test.go:467: 
            	Error Trace:	/home/runner/work/river/river/internal/notifier/notifier_test.go:467
            	Error:      	Should NOT be empty, but was 0xc000[29](https://github.com/riverqueue/river/actions/runs/8257741142/job/22588819288?pr=258#step:8:30)11[40](https://github.com/riverqueue/river/actions/runs/8257741142/job/22588819288?pr=258#step:8:41)
            	Test:       	TestNotifier/MultipleSubscribersStress
FAIL

https://github.com/riverqueue/river/actions/runs/8257741142/job/22588819288